The present invention provides novel substituted epoxidized hydroxystyrylaza compounds, as well as cured compositions prepared from said epoxy resins.
Preparation of unsubstituted epoxidized hydroxystyryl pyridines is taught by Yan, Pearce and Bulkin in Organic Coatings and Applied Polymer Science Proceedings, volume 46, pages 482-488 published by American Chemical Society (1981). These epoxy resins differ from those of the present invention as being higher melting, thus more difficult to process. A further difference is the poor thermal stability of the prior art unsubstituted styryl pyridine epoxy resins relative to the substituted styryl pyridine epoxy resins of the present invention. As a specific example, the epoxy resin of 2,6-di(4-hydroxystyryl)pyridine prepared by Yan and coworkers possessed a 175.3.degree. C. melting point and exhibited multiple cure exotherms at 289.5.degree., 305.degree., 334.degree. and 355.degree. C. (see page 486, Table 2 of the cited article). By way of contrast, the corresponding substituted styryl pyridine epoxy resin of the present invention, the epoxy resin of 2,6-di(3,5-dimethyl-4-hydroxystyryl)pyridine, unexpectedly provides a lower melting point of 158.degree. C. and a single cure exotherm at 315.degree. C. The poor thermal stability of the unsubstituted styryl pyridine epoxy resins of the prior art is exemplified by the epoxy resin of 2,4,6-tri(4-hydroxystyryl)pyridine which gelled and then exothermically decomposed to a black char during workup at 80.degree. C. (see Comparative Experiment C herein).
All of the aforementioned substantial and unexpected differences between the unsubstituted styryl pyridine epoxy resins of the prior art and the substituted styryl pyridine epoxy resins of the present invention are proposed to occcur as a result of reactive sites on the unsubstituted hydroxybenzaldehyde precursor to the unsubstituted hydroxystyryl pyridine used in the prior art epoxidation reaction. Specifically, these reactive sites lead to branched structure which elevates melting point, induces multiplicity in the thermal curing reaction and reduces processability. This branching can also sterically hinder phenolic hydroxyl groups which would otherwise be epoxidized. During workup of the resulting partially epoxidized product or subjection to some other source of thermal exposure, molecular motion increases thus making the phenolic hydroxyl groups available for reaction with epoxide groups also present in the molecules. Uncontrolled exothermic decomposition can then result. A specific example of the branching structure is shown for the hydroxystyryl pyridine precursor to an unsubstituted epoxy resin prepared using 2,4,6-trimethylpyridine and 4-hydroxybenzaldehyde: ##STR1##
A specific example of the lack of branching inherent to the compositions of the present invention is shown for the substituted hydroxystyryl pyridine precursor to a substituted epoxy resin prepared using 3,5-dimethyl-4-hydroxybenzaldehyde and 2,6-dimethylpyridine: ##STR2##
The non-branched structure of this product results from the blocking of both ortho positions to the phenolic hydroxyl groups with methyl groups. This prevents the competing aldehyde-aromatic ring condensation reaction which produces hydroxy functional dialdehydes responsible for the branching in the unsubstituted products: ##STR3##
The substituted epoxy resin compositions of the present invention contain styrylaza groups and are obtained by reaction of one or more substituted hydroxystyrylaza compounds as described by U.S. Pat. No. 4,600,767 which is incorporated herein by reference. Epoxidation of the substituted hydroxystyrylaza compound is completed in a conventional manner by reaction with an epihalohydrin with subsequent dehydrohalogenation with a basic-acting material and finally recovering the resultant styrylaza functional glycidyl ether product. The invention consists of the substituted epoxy resins containing styrylaza groups, whether or not cured.
The cured substituted styrylaza epoxy resins provide a combination of high mechanical strength and glass transition temperature. The enhancement of these properties is proposed to result from the self-reinforcing characteristic imparted by the anisodiametric styrylaza nucleus.